Web fabrication process



United States Patent 3,510,489 WEB FABRICATION PROCESS Arthur E. Graham and Ray L. Dueltgen, Lexington, Ky., William L. Mitchell, Austin, Tex., and Harry E. Sweazy, Winchester, Ky., assignors to International Business Machines Corporation, Armonk, N.Y., a corporation of New York No Drawing. Continuation-impart of application Ser. No. 419,240, Dec. 19, 1964. This application July 31, 1968, Ser. No. 748,920

Int. Cl. B29c 5/02, 5/08 US. Cl. 264-255 11 Claims ABSTRACT OF THE DISCLOSURE Magnetic recording webs are fabricated by praying a dispersion of binder resin and magnetic particles onto a mold surface and heating to remove the dispersant. Strong, dimensionally stable endless webs result by including in the formulation a polymerizable monomer, such as an ester of acrylic acid, which crosslinks when the sprayed layer is heated and cured.

Related application This application is a continuation-in-part of our application Ser. No. 419,240, filed Dec. 19, 1964, now abandoned.

Background of the invention Although there is much speculation in the prior art as to optimum processes to economically fabricate a magnetically responsive web, practical difficulties have resulted in somewhat cumbersome means being used. Generally, a supporting substrate of sufficient strength, such as polyethylene terephthalate, is used to provide a structural support upon which a mixture of ferromagnetic material separated and held by a binder is permanently deposited. The ferromagnetic material i the magnetically responsive element in the system. The strong substrate provides means to support, position, and move the ferromagnetic material. The binder serves basically three functions: it holds the ferromagnetic material to the sub trate, it separates the tiny bits of ferromagnetic material to thereby reduce eddy currents, and it isolates the individual bits of magnetic material from abrasive contact with a magnetic transducing head when the magnetic material is moved relative to a transducing head.

Because as discussed, at least three criterion must be met by the final form reached by the ferromagnetic material and binder mixture, the prior art has found itself limited in its manner of applying the mixture to the substrate. Even though the prior art suggests spraying and similar uses of fluid to apply the mixture of binder and ferromagnetic material, this assumes a fluid state of the mixture sufficient for such processing. In practice it was discovered that a mixture which could be brought to the desired fluid state by appropriate solvent later to be driven off or by similar means, resulted after the solvent was driven off in a magnetic layer which was deficient in some essential property. In particular, an excess amount of solvent u ed to make possible spraying of the mixture of binder and ferromagnetic material generally resulted in a deficiency in binder strength when 3,510,489 Patented May 5, 1970 the solvent was driven off or the mixture otherwise cured.

In practice in the past it has been found necessary to apply the mixture of binder and ferromagnetic material while in a highly viscous state created by a solution in which a limited amount of solvent was used. This rendered spraying and similar sophistications practically unavailable. In the prior art, a substrate such a polyethylene terephthalate was presented to a reservoir of the viscous mixture and the viscous mixture was coated on to the substrate under pressure. Since thi coating or extrusion process is cumbersome and requires closely dimensioned equipment, it was often found in the past to be economically unfeasible to form a magnetic web directly in the dimensions desired. Usually the magnetic layer and binder is coated on to bulk quantities of the substrate in a continuous process while the finished web is rolled into a bulk roll.

Yet, the industry often requires a relatively short con tinuous belt of the magnetic web. This was obtained by slitting the bulk Web to the desired width and cutting the bulk Web to the desired circumference of the continuous web to be fabricated. The ends of the length of the cut web piece were then butted together and joined with adhesive tape. Not only did this leave a minute discontinuity at the juncture point, but cumbersome and expensive steps are involved. As a. matter of fact, it has previously been common to accomplish the joining of the cut pieces and the application of the necessary adhesive manually.

Brief description of the invention This invention provides a process to create a magnetic web in which the limitations of extrusion are avoided in that a continuous closed web can be formed directly.

In accordance with the invention a polymer and a ferromagnetic material are dispersed in a low boiling organic liquid solvent or dispersant to provide a low viscosity sprayable mixture. A polymerizable monomer is also included in the mixture which is then sprayed into the surface of a mold which has the desired configuration of the finished article. After spraying, the sprayed material is subjected to heat in a curing step. At this time the organic liquid is evaporated and the unsaturated monomer acts to form linkages so that the final product can be very hard and tough even though the solution applied was low in viscosity. An initiator can be added to the mixture to facilitate the linking of the monomer.

Detailed description The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description and examples.

The binder resins which are preferred in the invention are vinyl resins such a polyvinyl chloride which is a sturdy material and is readily available.

Magnetic properties in the web prepared by the process of the invention are obtained by the presence, forexample, of acicular Fe O in the magnetic layer. Other ferromagnetic materials conventionally employed in magnetic recording materials can also be used. The amount of magnetic material used is determined by its oil absorbing properties which vary depending upon the particular magnetic material employed. Generally the amount of magnetic material ranges from about 35 percent to about 75 percent by weight of the magnetic layer. The binder resin, monomer, catalyst and other additions make up the remaining 65 to 25 percent by weight of the magnetic layer.

In order to provide sufficient strength to the finished article a polymerizable monomer is added to the binder resin-ferromagnetic material mixture. The monomer should be of relatively low volatility to prevent loss during the processing steps before it has reacted to form a crosslinked structure.

Preferred are polymerizable esters derived from aromatic carboxylic acids with unsaturated lower aliphatic alcohols for example, Santoset I (Monsanto Chemical Company) and polymerizable esters of acrylic acid such as 1,3 butylene dimethacrylate and trimethylol propane trimethacrylate.

The monomer is employed in amounts required to provide sufficient strength for the magnetic layer. Generally amounts in the range from about 5 to 50 percent by weight of the magnetic layer give satisfactory results.

To aid in the polymerizing of the monomer to a crosslinked structure, conventional free radical initiators such as organic peroxides, for example, t-butyl perbenzoate and di-t-butyl peroxide are added to the mixture.

If desired, other additives can be employed such as plasticizers, wetting agents and stabilizers, to maintain good dispersion of the magnetic material prior to curing a thixotropic filler can be added, for example, silica, carbon black, and bentone.

The components of the mixture are dispersed in a low boiling, volatile organic liquid dispersant or solvent such as ketones, halogenated aliphatic hydrocarbons. Examples of suitable liquids include acetone, methyl ethyl ketone, methyl isobutyl ketone, 1,1,1 trichloroethane, 1,1,2-trichloroethane, etc. The choice of solvent is not critical but it must be low boiling to faciiltate its removal during the curing step and solvents boiling at or below about 80 C. or below are preferred for this reason.

The amount of solvent must be sufficient to provide the mixture with a low viscosity for spraying. The amount generally will range between about 65 to 90 parts by weight of solvent per 100 parts of total mixture. Below 65 parts the viscosity is too high for spraying while above 90 parts the mixture is to thin to produce a strong, nonsagging coating.

The components are thoroughly mixed by conventional techniques so as to provide a homogeneous mixture. The order of mixing is not critical but it should be conducted so as to avoid the loss of dispersant or monomer prior to the spraying step and to assure good wetting dispersion of the magnetic material.

The mixture is sprayed onto a smooth mold surface. The surface can be the inner concave surface of a cylinder in which case the mixture is sprayed from a rotating disk in the center of the mold. The mold is heated to cure the layer during which time the dispersing liquid is removed and the monomer polymerizes to provide a strong cross-linked structure. The magnetic web, in the form of a continuous loop or belt is then stripped from the mold.

Although the web is self supporting, for added strength a second layer of polymer dispersed in a dispersing liquid can be sprayed over the magnetic layer. This layer can be either a polymer or a mixture of polymer and catalysed monomer. The polymer must be chosen so as to be compatible with the first layer so that curling and dimensional instability does not occur either when curing the webs or when they are used. While the same types of polymers and monomers used in the magnetic layer can be employed, other polymers such as polycarbonate resins can also be used.

EXAMPLE I A continuous magnetic web was formed, the web being four inches in width and about twelve inches in circumference. To fabricate the belt a mixture was compounded as follows:

Mixture A-Magnetic slurry Gms.

Acicular Fe O (IRN -Pr0duct of C. K. Williams, Inc.) 200.0 1,3' butylene dimethacrylate 298.0 Diethylene glycol monolaurate 2.4

Colloidal silica (Cab-O-Silproduct of Cabot Corporation) 36.0 Tertiary butyl perbenzoate 2.9 1,1,1, trichloroethane 2,000.0

Mixture A is milled in a pebble mill for seventeen hours. Non-ferromagnetic stones (Burundu-m) are used in favor of 'steel balls or other materials which could contaminate the magnetic property of the mixture. Milling is conducted to assure an intimate mixture of the components with the small bits of iron oxide thoroughly wetted by the mixture, and to divide the agglomerated iron oxide into their terminal size. Iron oxides are manufactured in their terminal size, but agglomerates form with later handling. Mixture A is formulated to approach an optimum between high mixing shear and adequate dispersing properties to hold the separations obtained. Mixture A is subsequently mixed with Mixture B.

In Mixture A the iron oxide is a well known material for magnetic recording formed in particles of a size small enough to fit together in a high density layer. The 1,3 butylene dimethacrylate is an unsaturated, polymerizable monomer which will be polymerized in situ during a curing step in accordance with the invention. The diethylene glycol monolaurate functions in the well known manner to wet the surface of iron oxide particles to thus disperse them. Tertiary butyl perbenzoate is an unstable peroxide type of compound which decomposes under the influence of heat to provide a free radical catalyst for polymerization of monomers. The 1,1,1 trichloroethane is a good dispersing solvent of mildly polar substances such as polyvinyl chloride.

The colloidal silica used has special and beneficial properties in this preferred embodiment. It is a thixotropic additive. In the invention a gel is formed with polar groups found in the mixture. The net etfect is as though a three dimensional wire mesh were placed within the liquid. The purpose of this is to slow the otherwise rapid settling of iron oxide under the influence of gravity. The result is greater uniformity of the product made.

In some production systems it may be feasible to perform a final mixing step just before application of a mixture containing iron oxide. In this event the colloidal silica is not needed in the Mixture A. An alternative would be to substitute carbon black for the colloidal silica for the purpose of obtaining a product which is black in color. Otherwise the brick red color of iron oxide dominates the color of the final product.

Mixture B Mixture B was formulated as follows:

Gms. Polyvinyl chloride 100.0 1,3 Butylene dimethacrylate 30.0 Dioctyl phthalate 30 .0 Tertiary butyl perbenzoate 0.3 Cadmium, barium, zinc heat stabilizer for polyvinyl chloride (Ferro 1237product of Ferro Chemical Co.) 3.0

Mixture B contains polyvinyl chloride, the resin used as a major component to provide strength and body to the final product. A smaller amount of 1,3 butylene dimethacrylate, the polymerizable monomer is also included in Mixture B, along with the catalyst, tertiary butyl perbenzoate. This polymerizable material is in Mixture B to assure intimate wetting of all parts of the mixture with this important component. Dioctyl phthalate is a well known plasticizer, which is used as such in this preferred form of the invention.

Mixture C, a sprayable, magnetic layer mixture is obtained by adding 74 grams of Mixture B to Mixture A after it has been milled. Appropriate amounts of the dispersing solvent are added as desired to thin the mixture to a consistency desirable for subsequent use with the processing apparatus in view of the product desired. This combination is pebble-milled for two additional hours to further mix the components. Because of the action of the colloidal silica, as discussed above, Mixture C can be sprayed in accordance with this invention with no substantial change in uniformity of the product, as long as 30 minutes to an hour after mixing.

Mixture E is the sprayable substrate mixture. First, Mixture D is formulated as follows:

Mixture D-Substrate Slurry Gms. Polyvinyl chloride (Blacar Pl716--product of Cary Chemical Co.) 100.0 Non-acrylic, polymerizable monomer (Santoset I, a mixture of phthalic acid esters of unsaturated aliphatic alcoholsproduct of Monsanto Chemical Company) 40.0 Barium cadmium zinc heat stabilizer (Mark KGB-product of Argus Chemical Co.) 3.0 Epoxidized soybean oil (Paraplex G61product of Rohn & Haas Co.) 3.0 Dibutyl tin dilaurate (Catalyst T12product of M & T Chemical Co.) 2.0 Di-t-butyl peroxide 0.8 1,1,1 trichloroethane 15.0

The materials of Mixture D are mixed in a Hobart mixer. To prevent loss of solvent, the major amount of solvent is added later. Thus, before Mixture D is used, Mixture D is diluted with one part by weight Mixture D to one part by weight 1,1,1 trichloroethane. The resulting combination is Mixture E, the sprayable substrate mixture.

A polymerizable monomer, Santoset I-product of Monsanto Chemical Co., is used in Mixture D. It will be observed that the monomer is not the 1,3 butylene dime'thacrylate used as a polymerizable monomer in Mixture A. The monomers could be the same. However, difficulty was experienced because of the evaporation of 1,3 butylene dimethacrylate from the outer layer during the curing of the mold, which will be fully described below. The monomer used in Mixture D improved the product of this preferred embodiment because it is not substantially volatile in the final mixture at the temperature of curing.

The barium-cadmium zinc stabilizer, the epoxidized soybean oil, and the dibutyl tin dilaurate act together as known in the prior art as heat stabilizers in Mixture D. Di-t-butyl peroxide is, of course, a free radical producing catalyst which will function as an initiator.

The thinned mixtures, Mixture C and Mixture E, are then sprayed'internally into a cylindrical mold answering the dimensions of the desired magnetic belt. The mold was a single piece of relatively thin stainless steel, having the general appearance of a common tin can with the top and bottom removed. The internal surface of the mold is the surface upon which the mixtures are coated. The mold material was stainless steel. The internal surface was highly polished and had an internal diameter of 4.08 inches and a width of 6 inches. As is noted above, the magnetic belt was intended to have a circumference of about twelve inches and a width of four inches.

Certain features having to do with the mold contribute substantially to good quality of the final product of this preferred embodiment. It has been found that the smoothness of the external surface of the final magnetic layer produced has a dramatic effect on the quality of recording. It may be desirable to further improve the magnetic qualities by use of a magnetic orientation step. A smooth mold of the nature required is achieved by coating the highly polished stainless steel mold with a silicone, specifically dimethylpolysiloxane. The silicone is a viscous, non-volatile liquid which wets steel, the material of the mold. This is commercially obtained packaged under gaseous pressure to form a spray unit. The silicone is sprayed on by hand from the hand carried can much as paint and air fresheners can now be sprayed from a can. The inside of the mold is then wiped with an optical polishing cloth until any apparent excess is removed. This leaves minute irregularities on the mold filled with silicone.

The silicone treatment need not be repeated with repeated use of the mold in practicing this invention. The silicone remains undisturbed through a large number of similar uses. This is true even when the cured webs are blown from the mold with air pressure. The beneficial effects are substantial. Noise and other vagaries in the magnetic properties of the magnetic belt as it interacts with a magnetic transducer are greatly reduced.

In the preferred embodiment a De Vilbiss disc spray unit (Gun Type MBC-Extension PMBX-520'l) was used. However, the relatively large amount of solvent included in Mixtures C and E reduced the viscosity of the mixtures to easily manipulated levels. A particular choice of spray apparatus was not critical. The spray apparatus used generates a disc of spray issuing at ninety degrees at all directions from the spray nozzle. The nozzle of the spray unit is moved on the long axis of the cylinder formed by the mold. As mentioned, the spray issues laterally, so the entire circumference of the mold opposite the nozzle is coated. To avoid splatter the spraying is accomplished with two passes of the nozzle. The substrate layer can be applied in a single pass of the nozzle. Mixture C was first sprayed on the mold. Immediately thereafter, the substrate mixture, Mixture E, was sprayed in larger amounts on the layer just sprayed. The mold including its now sprayed interior surface was then placed in a forced draft oven (Precision Scientific Model No. 625) preheated at the temperature of 370 degrees F. with the air vent set at 3. The mold was cured for eleven minutes. The hot mold was removed from the oven and quenched at room temperature in tap water.

The cured pieces are then stripped from the mold, trimmed where necessary, and tested for recording quality. A unitary magnetic recording belt is created having a thickness of 0.0030 in. and, of course, carrying magnetic material on one side. To determine how long to spray the Mixtures C and E, test runs were first made with the same equipment and pressures which would be used in production run. Sprayed layers were sprayed under recorded conditions, cured, and measured. This data was used in the production run to achieve a magnetic oxide layer of about 0.0005 in. and a substrate layer of about 0.0025 in. It should also be noted that the cylindrical mold is rotated to more evenly distribute the sprayed layers under the influence of centrifugal force.

It should be noted, however, that the magnetic layer was not designed to have the identical properties of the substrate layer. In particular, a relative deficiency of plasticizer in the substrate layer was intended to create a substrate layer of substantial hardness, whereas the increased amount of plasticizer and similar nonomers in the binder layer was intended to give the binder layer a degree of resilience which is beneficial to reduce impact and friction when a magnetic transducer rides along the magnetic layer.

The magnetic belt in which no magnetic orientation step had been conducted was tested for magnetic properties and found to have properties commensurate with those presently known.

EXAMPLE II A sprayable magnetic layer was prepared by mixing the components of the formulations listed below.

Parts by wt. Magnetic iron oxide: Acicular Fe O (CX5948 product of C. K. Williams Co. 100 Binding resin: Polyvinyl chloride (Blacar P17l6- Cary Chemical Co.) 28.4 Monomer: Trimethylol propane trimethacrylate (SR-350-Sartomer Resins Inc.) 11.3 Thickener: Carbon black (Raven 40Columbia Carbon) 6.0 Wetting agent: Castor oil (MPA 60Baker caster Oil) 15.0 Initiator: Di-t-butyl peroxide 0.30 Plasticiser: Dioctyl phthalate 5.6 Heat Stabilizers:

Barium and cadmium heat stabilizer (Mark KCBArgus Chemical Co.) 0.87 Paraplex (alkyd-glycol polyester G67 Rohm & Haas) 0.87 Dibutyl tin dilaurate (T12M & T Chemical Co.) 0.58 Dispersing liquid:

Methyl ethyl ketone 600.0

The carbon black in 300 grams of methyl ethyl ketone was mixed in an attritor for minutes. To this mixture was added the castor oil in 200 grams of methyl ethyl ketone and then polyvinyl chloride, trimethylol propane trimethacrylate, dioctyl phthalate and heat stabilizers in 75 grams of methyl ethyl ketone. The resulting binder mixture was mixed in the attritor for 60 minutes. Approximately /2 of the mixture was removed and stored and to the remaining portion there was added the magnetic iron oxide. Mixing was continued until the iron was throughly dispersed which took 14 hours. The stored half of the binder mixture was then added in 50 gram increments, with mixing, with about 2 minutes between each addition. The procedure took about 30 minutes. The catalyst in 25 grams of methyl ethyl ketone was then added and mixing continued for 60 minutes. The mixture was then sprayed onto the mold as described in Example 1 to provide a 0.5 mil dry cured layer. The coating was magnetically aligned by placing the mold and wet coating under the influence of a magnetic field of 1000 gauss for one minute while rotating the mold. The alignment is conducted to produce a recording belt having a reduced noise level.

A backing layer was then sprayed onto the magnetic layer which layer comprised 7.0 parts of a polycarbonate resin (Lexan 125General Electric Co.) in 93 parts of 1,1,2-trichloroethylene to a thickness to provide a dry layer 1.5 mil thick or a cured belt having a total thickness of about 2.0 mils. The layers were then heated at 110 C. by infrared radiation for 15 minutes to remove the volatile solvents and the mold was placed in an oven at 145 C. for 3 minutes to remove the remaining solvent. The mold was then placed in an oven at 250 C. for four min utes to cure and crosslink the monomer after which it was returned to the 145 C. oven for five minutes and placed in an oven at 80 C. for five minutes followed by air cooling at room temperature 25 C. for minutes. The purpose of the gradual cooling was to avoid shrinkage of the belt which would occur should the belt be cooled rapidly. A strong, dimensionally stable, endless magnetic 100p resulted.

EXAMPLE III The mixing, spraying, aligning and curing procedur s of Example 2 were repeated to produce a magnetic web using the following magnetic layer formulation and the same backing layer formulation used in Example 2.

Parts by wt.

Iron: Acicular Fe O (Cx5948product of C. K.

Williams Co.) 100 Binder resin: Polyvinyl chloride (Blacar P1716 Cary Chemical Company) Monomer: Mixed phthalate esters of unsaturated aliphatic alcohol (Santoset IMonsanto Chemical Company) The cured belt had excellent recording properties.

In summary, magnetic belts are created by an economical process having entirely satisfactory and improved characteristics which have no discontinuities or joints. The important quality supplied by the invention is the reduced viscosity of the mixtures used which permits economical fabrication without any sacrifice of ultimate characteristics of the finished web.

While the invention has been particularly shown and described with reference pref rred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

We claim:

1. The process of forming a continuous magnetic web consisting essentially of:

mixing a polymer, an unsaturated monomer polymerizable at the position of unsaturation, a free radical catalyst for initiating polymerization of said monomer, a volatile solvent to solvate said polymer, and small particles of ferromagnetic material, said small particles of ferromagnetic material being present in an amount of from about 35 to percent by weight of the weight of said mixture excluding said solvent, said monomer being present in an amount from about 5 to 50 percent by weight of the weight of said mixture excluding said solvent and said solvent being present in an amount of from about 65 to percent by weight of the total weight of said mixture including said solvent,

uniformly coating a mold having a smooth inner concave cylindrical surface on said surface with said mixture by spraying said mixture from points along the center line of said surface, said coating being applied in an amount suflicient to produce upon evaporation of said solvent a layer of about .0005 inch in thickness,

a heating the coating at an elevated temperature to cause said solvent to evaporate and to cause said monomer to be at least partially polymerized at said position of unsaturation by said free radical producing catalyst, gradually cooling said coating to ambient temperature, and

stripping said coating from said mold.

2. The process as in claim 1 in which, prior to said coating, the voids of said mold are filled with a viscous non-volatile liquid dimethylpolysiloxane which wets the surface of said mold.

3. The process as in claim 1 in which said polymer is polyvinylchloride.

4. The process as in claim 1 in which said monomer is a polymerizable ester of acrylic acid.

5. The process as in claim 1 wherein said monomer is a polymerizable ester derived from an aromatic carboxylic acid and an unsaturated lower aliphatic alcohol.

6. The process of claim 1 wherein said monomer is selected from the group consisting of 1,3-butylene dimethacrylate, and trimethylol propane trimethacrylate.

7. The process as in claim 1 in which said mixture also contains a thixotropic material sufficient to suspend said small particles for a substantial time.

8. The process of claim 1 including the step of spraying and drying a polymer-volatile solvent mixture on the magnetic web to serve as a web strengthening substrate, said substrate having a thickness from about 00015 to 0.0025 inch.

9. The process of claim 8 wherein said polymer is a polycarbonate.

10. The process of claim 8 wherein said solvent is at least a partial solvent for said magnetic web.

11. The process of claim 8 wherein said polymervolatile solvent mixture includes a polymerizable monomer and a free radical initiator for said monomer.

References Cited UNITED STATES PATENTS ROBERT F. WHITE, Primary Examiner K. .T. HOVET, Assistant Examiner US. Cl. X.R. 

